1. Field of the Invention
This invention relates in general to high frequency spread spectrum receivers and, in particular, to spread spectrum receivers of the frequency-hopping type. The invention relates especially to a technique and apparatus for automatic gain control (AGC) in a frequency hopping receiver.
2. Description of Prior Art
Frequency hopping is a commonly used technique to provide a secured communication system. Frequency hopping involves the changing of the radio carrier or center frequencies periodically to avoid detection or jamming. In systems employing very fast frequency hopping, the signal is transmitted at each frequency for a very short period such as 20 milliseconds.
For frequency hopping voice communication, the received signal strength must be estimated so that the AGC level can be set. The conventional AGC selection approach, which computes the average sampled energy of the hopping signal, is often used for voice radio. This approach works well in steady benign (non fading) environments with no interference. However, in a severe fading channel, and with the presence of interference over a portion of the hopping bandwidth, the averaging method is vulnerable to error due to the significant contribution of the interference signal to the sample average. The conventional method has also proven to be inefficient in severe slow fading channels such as the Rayleigh fading channel at high frequency (HF).
One approach is to use circuitry having very fast attack and release times in the AGC control circuit to enable the AGC circuitry to respond to each frequency hop. However, the analog circuitry providing sufficiently "fast" attack and release times is complex and expensive. Accordingly there is a need for improved AGC control in frequency hopping receivers to provide improved gain control in the presence of interference or fading.